The present invention relates to a method for inspecting microfine patterns for defects and foreign substances, the microfine patterns being formed on a substrate in a thin-film process represented by a semiconductor manufacturing process and a flat panel display manufacturing process. The invention also relates to an apparatus using such an inspection method.
As a conventional semiconductor inspection apparatus, JP-A-2004-55695 discloses a construction that uses a laser beam to check for defects in semiconductor photo masks and wafers. The light source is a laser and thus an illumination wavelength is a single wavelength.
JP-A-2004-87820 discloses a method and system to search and reference past data on inspection conditions of similar products and set new inspection conditions.
For example, a semiconductor wafer to be inspected has wiring patterns arranged in multiple layers, with an interlayer insulating film formed between the pattern layers for electric insulation. There are two types of the optical semiconductor wafer inspection apparatus, a bright field illumination type and a dark field illumination type. Both of these types perform inspections mainly by comparing images of dies on which patterns of the same design are formed. To enhance the inspection sensitivity of the image comparison system, it is desired that pseudo defects the apparatus user defines not be seen on the image. The pseudo defects include brightness differences between detected images corresponding to thickness differences between interlayer insulating films, and also grains. Generally, since the pseudo defects do not have adverse effects on electric characteristics of semiconductor devices, the apparatus user wants these pseudo defects left undetected. For this reason, it is unavoidable to raise a defect decision threshold for a difference image calculated from the image comparison so as to allow the pseudo defects.
The brightness differences between detected images that depend on the thickness differences between interlayer insulating films and which constitute the pseudo defects, become large as the illumination wavelength width narrows and small as it widens. That is, the effects that the thickness variations of the interlayer insulation films have on the brightness variations of the detected images increases as the wavelength width of the illumination light decreases. Therefore, to reduce the brightness differences between detected images caused by the thickness differences between interlayer insulating films, it is advantageous to widen the wavelength width of the illumination light. This makes it possible to reduce the defect decision threshold. As a result, the possibility of being able to detect fine defects of small grayscale depths becomes high, contributing to an enhanced level of detection sensitivity. Thus, in the image comparison-based inspection, when an illumination light of single wavelength, such as disclosed in JP-A-2004-55695, is used, the grayscale variations of the pseudo defects may increase degrading the inspection sensitivity. Further, on the wafer to be inspected there are various kinds of defects as well as pseudo defects that are preferably not detected.
Since grayscale variations of these defects and pseudo defects change with the illumination wavelength, it is advantageous in terms of enhancing the defect detection sensitivity to select a wavelength range for an illumination light that increases a contrast of defects and reduce brightness variations (grayscale depths) of pseudo defects. With the invention disclosed in JP-A-2004-55695, since a laser is used, the wavelength is a single wavelength and there is no room for wavelength selection. Therefore, when detecting defects by using a particular wavelength, as disclosed in JP-A-2004-55695, some kind of defects may get a large grayscale depth and some kind may fail to get a sufficient grayscale depth. So, to increase a detection rate for a wide range of defects, it is useful if a function to choose an appropriate wavelength for inspection is available.
To realize a higher inspection sensitivity, optical means to increase the grayscale depth of defects are necessary.
When a plurality of optical means are used to further increase the grayscale depth of defects, there are problems that finding optimum conditions takes time and that determining the optimum conditions is difficult.
Further, JP-A-2004-87820 discloses a method that sets new inspection conditions by searching past data on inspection conditions of similar products. This method, however, does not consider changing the illumination wavelength range as one of the inspection conditions according to the material of the wiring pattern of a specimen. In other words, it does not consider setting an inspection condition that best matches defects of various kinds and pseudo defects that preferably are left undetected, both present on the wafer.